1. Field of the Invention
The present invention relates to a differential multilevel optical signal receiver apparatus for receiving an optical signal modulated by differential multilevel modulation.
2. Description of the Related Art
As a technology for transmitting signals in an optical transmission system, phase modulation has been put to practical use widely. In phase modulation, data is transmitted by shifting the phase of a carrier wave in accordance with the transmitted data. In Quadrature Phase Shift Keying (QPSK), for example, “θ”, “θ+π/2”, “θ+π” and “θ+3π/2” are assigned to each symbol comprising 2-bit data, “00”, “01”, “11” and “10”, respectively. Here, “θ” is an arbitrary phase. A receiver apparatus can regenerate the transmitted data by detecting the phase of the received signal.
When increasing the transmission speed or distance of a transmission path, deterioration of an optical S/N ratio becomes a problem in the receiver apparatus. In recent years, research and development of differential multilevel optical modulation has advanced as a modulation method which enables the improvement of receiver sensitivity. In this description, an explanation is provided of an example of Differential Quadrature Phase Shift Keying (DQPSK) modulation representing the modulation. In DQPSK modulation, the phase of a carrier wave (“θ” “θ+π/2”, “θ+π” or “θ+3π/2”) is determined in accordance with a “difference” between a symbol value transmitted previously and a symbol value to be transmitted next. Therefore, when demodulating the DQPSK signal in the receiver apparatus, a phase difference between the two consecutive symbols is detected.
FIG. 1 is a diagram describing an example of a conventional DQPSK optical receiver apparatus. In FIG. 1, an optical splitter 101 splits an input optical signal and guides the split signals to interferometers 110 and 120. The interferometer 110 comprises an optical splitter 111, a 1-symbol delay element 112, a π/4 phase shifter 113, and an optical coupler 114. In the interferometer 110, an optical signal arriving at the optical coupler 114 from the optical splitter 111 via the 1-symbol delay element 112 interferes with an optical signal arriving at the optical coupler 114 from the optical splitter 111 via the π/4 phase shifter 113. The interferometer 110 generates a pair of complementary optical signals. In the same manner, the interferometer 120 comprises an optical splitter 121, a 1-symbol delay element 122, a −π/4 phase shifter 123, and an optical coupler 124, and generates a pair of complementary optical signals. Balanced photodiodes 131 and 132 convert the optical signals output from the corresponding interferometers 110 and 120 into electrical signals. The signals acquired from the balanced photodiodes 131 and 132 are equivalent to the transmitted data.
A configuration and operation of the DQPSK optical receiver apparatus shown in FIG. 1 is described in, for example, Patent Document 1 (Japanese publication of translated version No. 2004-516743 (WO2002/051041 or US2004/0081470)) in detail.
FIG. 2 is a diagram showing another example of a conventional DQPSK optical receiver apparatus. In FIG. 2, a pair of the optical signals output from an optical splitter 101, are guided to optical splitters 141 and 142. One of the output signals of the optical splitter 141 is guided to an optical coupler 114 via a π/4 phase shifter 113, and the other output signal of the optical splitter 141 is guided to an optical coupler 124. In the same way, one of the output signals of the optical splitter 142 is guided to an optical coupler 124 via a −π/4 phase shifter 123, and the other output signal of the optical splitter 142 is guided to an optical coupler 114. At that time, the transmission time period corresponding to an optical path from the optical splitter 101 to the optical splitter 142 is longer than that corresponding to an optical path from the optical splitter 101 to the optical splitter 141 by the time period of 1-symbol. Consequently, the 1-symbol delay elements 112 and 122 shown in FIG. 1 are realized.
A configuration and operation of the DQPSK optical receiver apparatus shown in FIG. 2 is described in, for example, by Patent Document 2 (WO2003/063515) in detail.
The amount of phase shift in the π/4 phase shifter 113 and the −π/4 phase shifter 123 need to be adjusted with high precision in order to control data error. For that reason, in optical receiver apparatus for receiving high-speed data in particular, as shown in FIG. 2, adjuster units 115 and 125 may be configured for adjusting the amount of phase shift in the π/4 phase shifter 113 and the −π/4 phase shifter 123. In the case the amount of phase shift in the π/4 phase shifter 113 and the −π/4 phase shifter 123 changes depending on the temperature, for example, the adjuster units 115 and 125 are heaters.
In Non-patent Document 1 (Michael Ohm, “Optical 8-DPSK and receiver with direct detection and multilevel electrical signals”, Advanced Modulation Formats, 2004 IEEE/LEOS Workshop on 1-2 Jul. 2004, Pages 45-46), for example, it is described how 8-DPSK (or 2n-DPSK where n is an integer) optical signal can be received by configuring a multilevel detection circuit for processing an electrical signal output, converted photoelectrically by a DQPSK optical receiver apparatus, as a multilevel signal. In addition, an optical signal modulated by DMAM (Differential M-ary Amplitude (shift keying) Modulation) such as a DQAM (Double Quadrature Amplitude Modulation) modulated signal can be received by using a multilevel detection circuit, using a similar technique to the reception of the 8-DPSK optical signal, after photoelectric conversion to an electrical signal.
In the configuration shown in FIG. 1, two 1-symbol delay elements (112 and 122) are required. Therefore, the configuration is not suitable for reducing the size of the optical receiver apparatus. Further, the 1-symbol delay elements 112 and 122 must be adjusted so as to have the same optical path length, and the adjustment is required in two lines. Therefore, the configuration is not favorable in terms of cost.
In the configuration shown in FIG. 2, the same function as that of the optical receiver apparatus shown in FIG. 1 can be provided with one 1-symbol delay element alone. However, in this configuration, the π/4 phase shifter 113 and the −π/4 phase shifter 123 have to be located close to each other in order to reduce the size of the optical receiver apparatus. For that reason, in an optical receiver apparatus comprising heaters, coolers or electrodes for electro-optic effects etc. as the adjuster units 115 and 125, thermal or electrical crosstalk occurs due to a spatial diffusion effect of the physical property, and thus there is a possibility that the amount of phase shift of the π/4 phase shifter 113 and the −π/4 phase shifter 123 can not be adjusted with high precision.